01 - Preheat oven to 375°F. Line a 12-cup muffin tin with paper liners or lightly grease.
02 - In a medium bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
03 - In a large bowl, whisk melted butter and sugar until well combined. Add eggs one at a time, whisking after each addition. Stir in milk and vanilla extract.
04 - Gently mix dry ingredients into wet mixture until just combined; avoid overmixing.
05 - Fold diced apples into batter until evenly distributed.
06 - Divide batter evenly among 12 muffin cups, filling each about three-quarters full.
07 - In a small bowl, combine flour, brown sugar, cinnamon, and salt. Cut in cold butter using a pastry blender or fingers until mixture resembles coarse crumbs.
08 - Sprinkle streusel topping evenly over each muffin.
09 - Bake for 20 to 22 minutes, or until a toothpick inserted into the center comes out clean.
10 - Allow muffins to cool in the tin for 5 minutes before transferring to a wire rack to cool completely.